38 Development of an Online Supporting System Flood Warning for Vu Gia Watershed, Quảng Nam Province, Vietnam: Conceptual Framework and Proposed Research Techniques Nguyễn Kim Lợi1, N
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Development of an Online Supporting System Flood Warning
for Vu Gia Watershed, Quảng Nam Province, Vietnam:
Conceptual Framework and Proposed Research Techniques
Nguyễn Kim Lợi1, Nguyễn Duy Liêm1, Phạm Công Thiện1,
Lê Văn Phận1, Lê Hoàng Tú1, Hoàng Thị Thủy1, Nguyễn Văn Trai1,
Trần Lê Như Quỳnh1, Lê Tấn Phúc1, Nguyễn Thị Huyền1, Nguyễn Thị Tịnh Ấu2, Nguyễn Thị Hồng3, R Srinivasan4*
1 Nông Lâm University, Hồ Chí Minh City 2
University of Technical Education Hồ Chí Minh City
3 VNU University of Science 4
Texas A&M University, USA
Received 7 February 2013 Revised 5 March 2013; Accepted 20 June 2013
Abstract: Vu Gia Watershed is located in the Central Vietnam where hurricanes pose a constant
natural threat to human lives and physical infrastructure Hydrological monitoring is considered as
an effective tool to respond to high floods by preventing and mitigating the losses The purpose of
this study was to conduct and perform hydrological modeling to determine the flood-prone areas
during the high rainfall season in the upstream Vu Gia Watershed The methodology involved:
hydro-climatic database building, a detailed Digital Elevation Model (DEM), a land use cover, and
a soil map of the basin With all these data, the SWAT model (Soil and Water Assessment Tool)
was used to predict discharge values These discharge values were used, along with the DEM, to
predict flood hazard areas in the downstream of Vu Gia Watershed floodplains This procedure
was made using the HEC-RAS model (Hydrological Engineering Center-River Analysis System)
The results show the exact location of areas with high, moderate and low risk, which are to be
flooded at specific high floods The results also provide the location in critical situation, so that an
early warning system can be located Additionally, as a part of this study, valuable information
about how to prevent and mitigate the affects of flood-related damage was provided to residents at
risk in the low land areas of the Vu Gia Watershed
Keywords: Flood warning, SWAT, HEC-RAS, Vu Gia Watershed, Quảng Nam Province
1 Introduction *
The Vu Gia Watershed, with an area of
about 466,128 ha, is located in mid-central
*
Corresponding author Tel.: 84-989617328
E-mail: ngkloi@hcmuaf.edu.vn
region of Vietnam, key economic zone of the Central region The geographical location is of the advantageous conditions for socio-economic development of the province
However, this is also the area that is seriously affected by natural disasters, and therefore has
Trang 2negative impacts on economic growth rate of
Quang Nam Province In recent years, under
changes of global climate and socio-economic
development; natural disasters in general, and
storm-flood in particular are increasing
abnormally and become more and more
damaging Considering different types of
natural disasters, the ones related to flood are
on top in incidence, severity and frequency of
occurrence, and they are also the types causing
most economic, social and environmental
damages According to recent five-year
statistics from 2003 to 2007, the losses due to
natural disaster in Quảng Nam Province are
estimated up to 6.26% of GDP In those years
with excessive rains and floods, losses can sum
up to 18-20% of GDP and severely crash both
human live and property This great natural
disaster’s losses need systematic study to find
out the cause and preventive measure to
mitigate the damage
Although there have been many studies on
the hazards of flood, there are remaining
limitations in-depth disciplinary scope and
study area Particularly, after the recorded
floods in late 1999, 2007, 2009, and the severe
drought in 2005, the calculation and evaluation
on the factors of flood and drought need to be
reconsidered In line with global climate change,
the variation of flow becomes more and more
extreme The disasters related to flood, occur
more frequent and cause more severe damages
The purpose to build flood warning system for Vu
Gia watershed is especially important
Hence, this research attempts to solve the
selected Vu Gia watershed in context of flood
warning system through the GIS-IT and integrating
SWAT and HEC-RAS models approach
This study aims to support farmers who live
in downstream Vu Gia watershed for preventing
flooding, the main aim in this investigation is how
to apply Geographic Information System (GIS)
and Information Technology (IT) and Soil and
Water Assessment Tool (SWAT) model and HEC-RAS model to build flood warning system for Vu Gia watershed, Vietnam The specific objectives of this study are as follows: (1) to determine vulnerability flood area and peak flooding in Vu Gia watershed; (2) to build the online website support information about hydrometeorology at real time; (3) to support farmer in vulnerability flood area by SMS message
2 Study area description
The Vu Gia watershed locates in the East of Truong Son Mountain Range, at latitude of
16o55'-14o55‟ North, longitude 107o15'-108o24' East The watershed is one of the largest river basins in the central coastal region as shown in Figure 1 The main slope direction of the basin
is north west - east south with average slope of 25.5% Upstream of the basin is a high mountain area with height of 1,700-2,045m Mountain chains create arc which bars north, west and south parts of the basin Downstream
of the basin is a plain next to the sea
Fig 1 Vu Gia watershed map
Trang 3The Vu Gia watershed in Quảng Nam
Province with an area of 10,350km2 is the most
vulnerable place to storm, flood Annually,
those disasters have caused losses estimated up
to thousands of billions VND and losses of
people’s life Due to this reason, there have
been a lot of programs and projects
implemented recently and they could bring
valuable results not only in term of science but
also in term of practicality for mitigating the
effects of flow-related disasters like flood and
drought in Quảng Nam
The climate in the area is tropical monsoon
and characterized by a wet and a dry season
The wet season starts from July and ended in
December, but it may extents to January in the
eastern part of the province in some area,
during the east west monsoon The dry season
covers the remaining months of the year
Average temperature of year is 20-210C and
does not vary much between moths of year
Relative humidity is generally high in the rainy
season (April to October) and low during the
dry season (November to March) The mean
annual average humidity is recorded as 86.5%
with the highest value of 97% in September and
lowest of 10% in March Therefore, in
agriculture and aquaculture are facing critical
conditions that can cause diseases or illness for
plant and animal The annual rainfall is about
3,600mm More than 80% of the rainfall is
concentrated in the wet season Heavy rains
usually come in July and October making the
water level in the rice fields near the stream rise
quickly causing short-term floods
The rainy season in Quang Nam Province is
similar and can be divided into three distinct
parts, i.e early rainy season (June to August);
mid rainy season (September to November);
and late rainy season (December to January)
Future climate scenario was analyzed for the
Quang Nam Province, based on data from
future climate projection from SEA-START
Research Center, which shows trend of warming
temperature and increasing annual precipitation in the area (Figure 2 and 3), which may cause higher flood risk and landslide in the area
3 Data and methology
3.1 Data
The necessary data for this investigation were collected within and outside the study area
as follows: topographic map at scale 1:50,000, land use map, soil map, climate data The data were processed using GIS software, ArcSWAT software, and HEC-RAS model
3.2 Methodology
3.2.1 Brief description of SWAT model The Soil and Water Assessment Tool (SWAT) has been widely applied for modeling watershed hydrology and simulating the movement of non-point source pollution The SWAT is a physically - based continuous time hydrologic model with Arcview GIS interface developed by the Blackland Research and Extension Center and the USDA-ARS (Arnold
et al., 1998) [1] to predict the impact of land management practices on water, sediment, and agricultural chemical yields in large complex basins with varying soil type, land use and management conditions over long periods of time The main driving force behind the SWAT
is the hydrological component The hydrological processes are divided into two phases: the land phase, which control amount of water, sediment and nutrient loading in receiving waters; and the water routing phase which simulates movement through the channel network The SWAT considers both nature sources (e.g mineralization of organic matter and N-fixation) and anthropogenic contributions (fertilizers, manures and point
sources) as nutrient inputs (Somura, H et al.,
2009) [2] The SWAT is expected to provide
Trang 4useful information across a range of timescales,
i.e hourly, daily, monthly, and yearly
time-steps (Neitsch et al., 2002) [3]
3.2.2 Hydrologic Engineering Center River
Analysis System (HEC-RAS) Model
HEC-RAS is a computer program that models
the hydraulics of water flow through natural rivers
and other channels The program is
one-dimensional, meaning that there is no direct
modeling of the hydraulic effect of cross section
shape changes, bends, and other two- and
three-dimensional aspects of flow The program was
developed by the US Department of Defense,
Army Corps of Engineers in order to manage the
rivers, harbors, and other public works under their
jurisdiction; it has found a wide acceptance by
many others since its public release in 1995 year
3.2.3 The Flood Warning System
The research started with the data collection process This consists of obtaining a current land use cover from a land use map provided by Quảng Nam Department of Natural Resources and Environment The soil and climate data bases were built using data from local government agencies and previous studies Both were transformed and edited to be used as input files for the SWAT model Rain gauges data were collected from all of four automatic weather stations distributed over the studied watershed The most time consuming work was
to build the four automatic weather stations Digital Elevation Model (DEM) for the Vu Gia watershed was collected from government agency Contour curves (20-meter) were digitized to complete a DEM for the entire area
yi
c)
Fig 2 Average maximum (a, b) - minimum (c, d) temperature at present and predicted values for the future
in Quảng Nam Province (Source: SEA-START, 2010)
Trang 5Fig 3 Average annual rainfall during present time and future in Quảng Nam province
(Source: SEA-START, 2010) Further, land use, soils, and climate data
were adapted with field data and information
collected from local agencies In order to
predict stream flow patterns, SWAT
simulations were ran using the soil, climate,
DEM and land use dataset
With the complete DEM, flood plains and
channel geometry features were mapped using
ArcGIS software and its 3D Analyst extension
River flow direction was also determined to
further use it as a model input variable
Cross sections are perpendicular lines to the
flow direction Their width varies depending
upon channel geometry and floodplain
configuration These cross sections were
calculated for the valley floodplains subjected
to frequent inundation events
Information from channel geometry and
SWAT-generated discharges values were used
to generate HEC-RAS channel flows HEC is a
very sophisticated computer program to model
water surface profiles from corresponding
discharge values The HEC-RAS model
calculates water surface elevations at all
locations of interest for given values It uses
Bernoulli equation as below (Equation 1) for
subcritical flow at each cross section (Bedient
and Huber, 2002):
(1)
where:
WS 1 , WS 2: elevation of water surface at each cross section;
V 1 , V 2: mean velocity;
α1, α2: velocity coefficient;
g: gravitational constant;
h e: energy head loss
All data were analyzed and processed using the above mentioned software and procedures Water surface elevations predicted with HEC-RAS model were used as input to generate the flood area coverage This information allowed
us to visualize where the high hazard areas might be located The general methodology was shown in Figure 4
As a last step, a vulnerability analysis workshop was conducted in a set of community meetings, in which at risk residents expressed their opinions on what they though it represented
a risk for their life Around 50 families were interviewed in the workshop using Participatory Rural Appraisal (PRA) method Specifically, the PRA method in combination with field visit was conducted in Dai Loc District to collect information for an general picture of the district regarding concerns in livelihood in relation with natural disasters; and adaptation capacity of local people to the new context
Trang 6dg
Fig 4 Structure of online supporting system for flood warning in Vu Gia watershed.
4 Main results
4.1 Model calibration and validation
The SWAT simulations were conducted for
a ten year period (2000-2009) Calibration of
SWAT was performed for years 2000 - 2003
using data from Vu Gia River basin, while the
data from the years 2004 - 2009 were used for
model validation Both graphical and statistical
approaches were used to evaluate the SWAT
model’s performance The statistical results of
the model performance for both calibration and
validation periods are summarized in Table 1
Figures 5 represents comparison of simulated
and observed water discharge during the
calibration and validation years at Thanh My
station, the figure clearly indicates that
simulated water discharge reasonably match the
observed water discharge most of the time
except for November 2000 and December 2007,
the model underestimated the water discharge
And in September 2002 and September 2008
the model overestimated the flow for Thanh My monitoring station
Table 1 Model performance for water
discharge simulation
Value
R2 NSI
Calibration (2000-2003) Monthly 0.61 0.68 Validation
(2004-2009) Monthly 0.68 0.73
Fig 5 Comparison of simulated and observed water discharge during 2000 - 2009 period
at Thanh My monitoring station
Trang 74.2 Online decision support system (DSS) for
flood warning
The online flood warning system has been
started in Vu Gia watershed since 2012 The SWAT
simulated and observed water level in 2012 year
at Thanh My station is shown in Figure 6 and
website http://gislab.hcmuaf.edu.vn/add_data/
The output water level from SWAT model was
automatically transfered to HEC-RAS model
(http://gislab.hcmuaf.edu.vn/output/Tmp1.Tmp)
The map of flood risk areas on October 24, 2012
is shown in Figure 7 and the residents who live in
flood areas were received a SMS message from
the system Based on the real time information
from flood warning system, the local government
(Quảng Nam Province) will make decision to
farmer who lives in risk area to response to
flooding The WebGIS online DSS for warning
was shown at http://gislab.hcmuaf.edu.vn/vugia/
Fig 6 The simulated and observed water level in
2012 at Thanh My monitoring station
Fig 7 Map of flood risk areas in Vu Gia watershed
on October 24, 2012
5 Conclusions
This research is just the first step to apply SWAT and HEC-RAS models in Vu Gia watershed The SWAT model performed well
in simulating the general trend of water level at watershed over time for secondly, hourly, daily, monthly time intervals This paper provides an insight of how the HEC-RAS model can be a useful tool for providing important information about river flow fluctuations affected by extreme rainfall events Future studies are needed to evaluate with more detail each land management practice Work is still in progress to improve SWAT and HEC-RAS data bases to Vu Gia watershed, Quảng Nam Province, Vietnam
6 Acknowledgements
The authors would like to thank Ministry of Science and Technology, Vietnam (MOST) for providing funded through “Online Supporting System Flood Warning for Vu Gia Watershed, Quảng Nam Province, Vietnam” project
References
[1] Arnold, J.G., Srinivasan, R., Muttiah, R.S and Williams, J.R 1998 Large area hydrologic modeling and assessment Part I: model development J American Water Resources Association 34: 73-89 [2] Neitsch, S.L., Arnold, J.G., Kiniry, J.R., Srinivasan, R and Williams, J.R 2002 Soil and Water Assessment Tool User’s Manual, version 2000 GSWRL Report 02-02, BRC Report 2-06 Temple, Texas, USA [3] Somura, H., Hoffman, D., Arnold, J.G., Takeda, I and Mori, Y 2009 Application of the SWAT model to the Hii River Basin, Shimane Prefecture, Japan In Soil and Water Assessment Tool (SWAT) Global Applications World Association of Soil and Water Conservation Special Pub No.4